From mboxrd@z Thu Jan 1 00:00:00 1970 From: "Phil Endecott" Subject: waitpid return value when there are no children Date: Mon, 14 Jan 2008 17:19:08 +0000 Message-ID: <1200331148207@dmwebmail.japan.chezphil.org> Mime-Version: 1.0 Content-Type: text/plain; format="flowed" Return-path: Sender: linux-man-owner-u79uwXL29TY76Z2rM5mHXA@public.gmane.org To: mtk.manpages-Re5JQEeQqe8AvxtiuMwx3w@public.gmane.org Cc: linux-man-u79uwXL29TY76Z2rM5mHXA@public.gmane.org List-Id: linux-man@vger.kernel.org Hi Michael, man waitpid says under RETURN VALUE: waitpid(): on success, returns the process ID of the child whose state has changed; on error, -1 is returned; if WNOHANG was specified and no child(ren) specified by pid has yet changed state, then 0 is returned. There is a subtle case when the child has no children at all and you call waitpid(-1,NULL,WNOHANG). To me, the existing wording implies that it will return 0. In fact it returns -1 and ECHILD, as I have discovered the hard way :-(. The SUS documentation is more explicit about this, e.g. http://www.opengroup.org/onlinepubs/007908799/xsh/wait.html: "If waitpid() was invoked with WNOHANG set in options, ****it has at least one child process specified by pid for which status is not available****, and status is not available for any process specified by pid, 0 will be returned. Otherwise, (pid_t)-1 will be returned, and errno will be set to indicate the error." I suggest the following, which is based on your existing text: waitpid(): on success, returns the process ID of the child whose state has changed; else if WNOHANG was specified and child(ren) specified by pid exist but have not yet changed state, then 0 is returned; else an error has occurred and -1 is returned. Regards, Phil.